<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title>第69个CSS页面</title>
    <style>
        img {
            position: absolute;
            left: 0px;
            top: 0px;
            z-index: -1;
        }
    </style>
</head>
<body>

<h1>这是一个标题</h1>
<img src="http://www.w3school.com.cn/i/eg_mouse.jpg" alt="No picture" />
<p>默认的z-index是0.z-index = -1具有更低的优先级</p>
</body>
</html>